In a common batchwise production method of polyamide using a nylon salt or its aqueous solution, the aqueous solution of nylon salt is heated under pressure in a single reaction vessel to allow the polymerization to proceed in a homogeneous phase while preventing the diamine component from escaping by evaporation. After fixing the diamine component, the water vapor in the system is gradually released to reduce the pressure finally to atmospheric pressure or reduced pressure, thereby completing the polymerization. In this method, since an aqueous solution of nylon salt with a concentration of about 50 wt % is generally used, a large amount of solvent water and the condensation water eliminated by condensation must be removed. Therefore, a countermeasure should be taken against various disadvantages, such as foaming, solidification of polymer due to the evaporation latent heat of water, heat degradation of polyamide which is adhered to the inner wall of reaction vessel due to a large change of liquid level during the reaction. In addition, this method requires a large amount of heat energy for removing a large amount of water and the single batch yield of polyamide is low. Therefore, this method involves many technical and economical problems. These problems can be fairly solved by the production method wherein the nylon salt is supplied as the raw material (Patent Documents 1 and 2). However, the proposed method requires additional steps for isolating and purifying the nylon salt, reducing the production efficiency.
Patent Document 3 discloses a polymerization method without using a nylon salt or its aqueous solution, in which the reaction is allowed to proceed by adding a diamine component containing a small amount of water dropwise to a dicarboxylic acid component at 220° C. under atmospheric pressure. Patent Documents 4 and 5 disclose methods wherein the reaction is allowed to proceed by adding dropwise a diamine component to a dicarboxylic acid component under atmospheric pressure. Although technically and economically advantageous, these methods involve a problem attributable to the addition of the diamine component to the dicarboxylic acid component under atmospheric pressure.
Since the molten dicarboxylic acid component is sublimable, the sublimated dicarboxylic acid component deposits on the ceiling of the polymerization apparatus. Also, the sublimated dicarboxylic acid component adheres to the inner wall of pipes connected to the upper portion of the polymerization apparatus, for example, the inlet for adding additives, inlet for adding the diamine component, and the pipe for introducing the vapor mainly containing the steam of condensation water which is eliminated by the polymerization reaction from the reaction tank to a partial condenser, and further adheres to the inside of the partial condenser. Most of the adhered sublimate of the dicarboxylic acid component is washed away in the polymerization step by the dissolution in the condensation water eliminated by the polymerization. The dicarboxylic acid component sublimates when the polymerization apparatus contains only the molten dicarboxylic acid component and also during the addition of the diamine component if the fixation of the dicarboxylic acid component is insufficient.
The sublimate of the dicarboxylic acid component adhered to the polymerization apparatus reacts with the diamine component brought by the vapor of the condensation water eliminated by polycondensation, thereby forming nylon salt or oligomer. As compared with the salt of m-xylylenediamine and a dicarboxylic acid, the salt of p-xylylenediamine and a dicarboxylic acid component is less soluble in water. Therefore, the amount of the salt which does not dissolve in the condensation water increases as the content of p-xylylenediamine in the diamine component increases. The amidation of the salt proceeds during the repeated batchwise productions to form oligomer, thereby further reducing the solubility in water. The adhered matter is subjected to a long-term heat history. If the adhered matter falls into the polymer during its production, the resultant final product, such as film, bottle, and monofilament, may suffer quality defects, such as gelation. Of the parts of the reaction apparatus, the nylon salt and oligomer adhere and accumulate most exceedingly in the pipe which introduces the vapor mainly containing the condensation water eliminated by the polymerization from the reaction tank to the partial condenser and in the partial condenser. If continuously accumulated, the pipe and the partial condenser are clogged, making it difficult to continuously repeat the batchwise production. In the production of polyamide from a diamine component and a dicarboxylic acid component, it is very important to control the mole balance to achieve the desired degree of polymerization. However, the amount of the accumulated salt and oligomer in the reaction tank varies batch to batch, making it difficult to precisely control the mole balance. Thus, the method of adding the diamine component to the dicarboxylic acid component has many disadvantages to producing polyamide with uniform and good quality.
Patent Document 6 discloses to add a whole amount of diamine component to a dicarboxylic acid component in an extremely short time and allow the reaction to proceed under pressure. The proposed method involves various disadvantages, which are attributable to the addition of a whole amount of diamine component in an extremely short time. Since a large amount of condensation water is eliminated in a short time, a countermeasure should be taken against foaming, change of liquid level, solidification of polymer due to the evaporation latent heat of water, and escape of monomers by evaporation. Particularly, a high pressure is needed. Therefore, it takes a long time to reduce the pressure for allowing the reaction to proceed, because the pressure should be reduced while preventing foaming. During the operation of reducing the pressure, the polyamide is exposed to high temperature for a long time, this promoting the oxidative degradation of polyamide molecules to cause yellowing. In addition, a large amount of heat energy is required in a short time for removing a large amount of the condensation water which is eliminated in a short time and maintaining the reaction system at a temperature enough to keep the whole reaction system in fluid condition, thereby requiring a heating means excessively large in view of the amount of the produced polyamide. Thus, the proposed method involves many technical and economical problems.
Patent Document 6 further discloses a production method of polyamide by the polycondensation of a dicarboxylic acid component and a diamine component under pressure, in which after completing the polycondensation under pressure, the pressure is decreased to allow the polymerization to further proceed. In this method, the polyamide is exposed to high temperature until the pressure is lowered to the desired level and the deterioration of the polyamide molecule by oxidation is promoted to cause yellowing. In contrast to the proposed method, it is recommended to allow the degree of polymerization to quickly reach the desired level after the whole diamine component is added to the dicarboxylic acid component.
The polycondensation of the dicarboxylic acid component and the diamine component under pressure involves a problem of taking a long time for decreasing the pressure while preventing the foaming. To shorten the process time after completing the addition of the diamine component, it has been known to control the inner pressure of the reaction system so that the pressure at the time the addition of the diamine component is completed is lowered. However, in the polycondensation under a constant pressure, to prevent the adhesion of the nylon salt and oligomer to the reaction system and effectively wash away the adhered matter with the condensation water eliminated by the polycondensation of the diamine component and the dicarboxylic acid, the polycondensation should be conducted under the lowest pressure enough to keep the effect of preventing the adhesion or under a controlled inner pressure of the reaction system. Patent Document 6 describes nothing about a method wherein the pressure is reduced while preventing the escape of monomers by evaporation during the polycondensation, i.e., during the addition of the diamine component to the dicarboxylic acid component.
Patent Document 7 discloses a method in which a diamine component comprising m-xylylenediamine and p-xylylenediamine is added to adipic acid, while decreasing the concentration of p-xylylenediamine in the diamine component in a later stage of the reaction. The proposed method uses different diamine components containing the ingredients in different proportions, thereby increasing the number of equipments to be used. In addition, the diamine component being adding is changed to another during the reaction, this complicating the operation. Thus, the proposed method cannot be said as an efficient method.
Therefore, it has been demanded to provide an efficient method of producing polyamide with uniform properties by the polycondensation of a diamine component comprising p-xylylenediamine and a dicarboxylic acid component.